1. Field of the Invention
This invention relates generally to equipment for feeding sheets, one at a time, from a stack in a hopper, and more particularly to a sheet feeder incorporating a second discharge stage that receives sheets serially from the sheet feeder and properly registers and aligns the sheets for precision placement on a moving web.
2. Discussion of the Prior Art
Longford International, Ltd. has, for several years, been marketing its Model OS700 surge feeder to the offset printing industry for feeding label stock to the input pinch point of a web press to create multilayer labels. The OS700 machine comprises a sheet feeder for dispensing sheets, one at a time, from a supply stack and a discharge conveyor for aligning and registering the individual sheets for accurate placement on a moving film web at a rate of about 10 per second.
The discharge conveyor of the OS700 machine comprises a pair of parallel, endless chains deployed about spaced-apart drive shaft sprockets proximate the discharge end of the sheet feeder and a pair of spaced-apart driver or idler shaft sprockets proximate a pinch point of the film web transport. Each of the endless chains carries a plurality of regularly-spaced lugs along the length of the chains. The lugs attach to the chain by a hinge mechanism that allows a pair of transversely aligned lugs (one on each chain) to drop straight down in unison just before reaching the idler shaft sprockets. That is to say, the lugs do not simply project normal to the chain as they round the idler shaft sprockets. This is to ensure that the lugs do not strike and damage the sheets being fed onto the downstream film web. The mechanism for effecting the desired downward movement of the chain lugs comprises several parts and is relatively expensive.
In operation, as the sheets are ejected from the sheet feeder onto the discharge conveyor, the lugs engage the trailing edge of the sheets and push the sheets along their path of travel. Because the lugs on the Longford machine effectively push the product as its trailing edge, it is unable to handle products that have features, such as tails or tabs that extend from the product's trailing edge.
Our invention is designed to be an improvement over the discharge conveyor used on the Longford OS700 surge feeder in that it eliminates the need for lugged chains for achieving registration and alignment of the sheets. It especially eliminates the complex hinging mechanism needed in the Longford OS700 machine to cause the lugs on adjacent chains to drop down in unison to avoid damage to the sheets being fed as they move from the discharge conveyor stage to the pinch point of the web press. Furthermore, in our invention lugs do not push against a product's trailing edge so our machine is able to handle products that the Longford machine cannot.